Foamy viruses (FVs; ... known as spumaviruses or spumaretroviruses) are classified as one of the genera of the Retroviridae. The FVs, first described in 1954(18) and isolated in 1971(1), have been rather neglected relative to their better known retroviral cousins, the type C oncoviruses and lentiviruses. Nonetheless, detailed analyses of this group during the past few years have revealed surprising, and in some cases remarkable, differences from all other retroviruses. In fact, in some respects their replication bridges the gap between retroviruses and the only other family of vertebrate reverse transcriptase (RT)-encoding viruses, the Hepadnaviridae.